Identifying Electronic Doorway States in the Secondary Electron Emission of Layered Materials

Abstract

Secondary electron emission induced by low-energy electron impact (<200 eV) plays a central role in electron-surface interactions and surface-sensitive technologies. However, despite decades of research, the underlying mechanisms are not yet fully understood, particularly at low kinetic energies where the spectrum is often as- sumed featureless. Coincidence spectroscopy, i.e. detecting correlated pairs of scattered pri- mary and secondary electrons, has challenged this view by revealing well-defined spectral fea- tures. For example, the prominent 3.3 eV “X peak" in graphite [1] was explained through co- incidence spectroscopy as a two-step process: plasmon excitation followed by hybridisation of interlayer states [2]. Building on this, we investigate how low- energy electron emission evolves as graphite re- duces from a bulk layered system to truly two- dimensional materials. We present coincidence measurements of secondary electron emission by 173eV primary electrons from highly oriented pyrolytic graphite (HOPG) and quasi-freestand- ing bilayer and single-layer graphene. Surpris- ingly, our results reveal layer-number-dependent features: HOPG shows the strong 3.3 eV X-peak resonance, bilayer graphene exhibits a less in- tense feature at 7.7 eV, while single-layer gra- phene shows no pronounced resonance (Fig. 1). Combined with density functional theory calculations, we identify these structures as Feshbach-type resonances: quasi-bound above- vacuum states that couple to free-electron states in vacuum and act as intermediate doorway states in the emission process. The existence and intensity of these resonances are highly sensitive to interlayer interactions, where the pronounced 3.3 eV resonance emerges only for systems with sufficient number of layers. These results establish coincidence spectros- copy as a powerful tool for disentangling second- ary electron emission pathways and demonstrate that the low-energy regime provides a sensitive probe of unoccupied electronic band structure in low-dimensional materials. As an outlook, we will also discuss recent energy- and k-resolved studies offering complementary insights into emission dynamics.